“People are quite surprised, but we did no conventional cardio the entire time we worked out together,” her trainer Paulette Sybliss told People. Instead, they focused on intense weight training.

Her weight-training regimen not only got Jackson her pre-baby body back, but also got her fit enough to perform. “She sings and dances. She does them together—she doesn’t do one or the other,” says Sybliss.

Sybliss points out that with cardio, calorie burn stops once you step off the treadmill, but with weight training, the after-burn continues for much longer. "My goal was, every time she’s not with me, I need her body to be working.”

Just eight weeks after giving birth, the 51-year-old singer started working out with Sybliss at least four days a week for 45 minutes. According to the trainer, Jackson’s best workout was her one-arm rows, which toned and sculpted her shoulders and arms.

She says that Jackson’s favorite thing to do was back work. As for her least favorite? Legs. Sybliss notes that the bigger the muscle group, the more physical work it takes, so leg workouts were especially intense, but that made it especially effective for burning calories.

Now that Jackson is in shape and performing again, Sybliss will continue to work with her to keep her healthy, fit and strong.